1. Field of the Invention
This invention generally relates to magnetic compass compensation systems, and more specifically, to a compensation apparatus suitable for use in highly ferrous land vehicles such as military armored personnel carriers and tanks.
2. Description of the Prior Art
Various types of North finding or seeking devices for navigable craft which employ compensation systems to eliminate the craft's permanent and induced magnetic fields are well known in the prior art, many of these devices being specifically designed for use in aircraft. These devices basically employ an Earth's magnetic field sensor for sensing the horizontal components thereof and a mechanical or electronic compensation unit attached to or electrically associated therewith which may be adjusted during a compensation swing to remove the disturbing effects of the vehicle's permanent and induced magnetic fields. Without such compensations, the vehicle's own magnetic field disturbances would make the readings of the magnetic sensor useless as a navigation aid.
In most magnetic compass applications, such as in marine vessels and aircraft, the primary disturbance or distortion of the earth's magnetic field in the vicinity of the magnetic compass sensor or indicator, i.e, the local earth's field, is produced by so called "hard iron" in the vehicle. This hard iron effectively produces a permanent magnetic field of its own which in combination with the earth's field distorts the latter so that the compass reading is in error. The hard iron, being fixed in the craft can be easily and conventionally compensated by determining through a compass swing, its horizontal components which may be "bucked out" or reduced to zero by generating equal and opposite magnetic field components in the vicinity of the compass. Since the hard iron or permanent disturbing field is fixed in the craft and is uninfluenced by the earth's field, the compensation is effective for all craft headings and attitudes. A secondary disturbance or distortion of the earth's magnetic field in the vicinity of the magnetic compass sensor is produced by the so called "soft" iron in the vehicle. This soft iron effectively produces a disturbance or distortion of the local field caused by the magnetic field induced in the soft iron elements of the vehicle by the earth's magnetic field. Therefore, when the vehicle changes attitude relative to the field direction of the earth, the magnitude of the field induced in the vehicle's soft iron also changes due to the change in the angle of incidence of the earth's field thereon. Thus, the compensation for the induced or soft iron error in the local earth's field is a much more difficult problem. As a matter of fact, many compass system installations, particularly in aircraft do not attempt to compensate for the soft iron error produced by significant attitude changes due to its temporary nature during normal flights.
Prior workers in the field of compass compensation have proposed compensation schemes which attempt to compensate for the induced field errors in a compass system, such as illustrated by Tolles in U.S. Pat. No. 2,692,970. This patent addresses the underlying theoretical aspects of such soft iron compensation for aircraft. However, in the Tolles scheme, the magnetometers associated with the compensation apparatus must be so located and fixed to the aircraft's vertical and transverse axes that they pick-up only corresponding components of the earth's field free of the hard iron and soft iron disturbances, since it is these values that are resolved as functions aircraft pitch and roll attitudes to provide the hard and soft iron compensation fields at the compass magnetometer. In general, the Tolles arrangement might be useful in highly ferrous land combat vehicles such as tanks if the pick-up magnetometers could be mounted on a high mast far removed from the metal of the tank. This may be and usually is unacceptable in modern tank warfare.
The present invention, therefore, results from a requirement that the primary magnetic field sensor or navigation system compass sensor be mounted within the protective armor of the vehicle. Further requirements included a provision for compass accuracy within a predetermined small error (e.g. .+-.3.degree.) not only when the vehicle or tank is on level ground but also when it is on uneven ground resulting in tilts as great as .+-.15 to .+-.20 degrees in pitch and roll.